It is highly desirable to produce a compact, lightweight, efficient, low-profile, high-gain, broadband antenna for use in wireless communications. Presently, antennas encompassing all of these qualities are not available. Usually, antenna design dictates that a trade off is necessary between size, bandwidth and efficiency. Recognition of the trade off has resulted in several prior art design approaches for antennas.
A reflector antenna, commonly a parabolic reflector, uses a horn radiator to illuminate its aperture. The shape of the reflector causes it to redirect energy fed to it by the horn in a high gain directional beam. Unfortunately, a horn-fed reflector is inefficient and bulky. Illumination of the reflector always results in either overspill or under utilisation of available aperture to avoid overspill. Typical efficiencies that can be achieved by a reflector antenna are 60%. Large overall size results from a boom supporting the horn and the reflector.
Another approach to antenna design uses an array of microstrip patches or another form of printed radiator. Such antennas are low-profile, as the depth is only a thickness of an antenna substrate. Arrays of microstrip patches group many low gain elements together, each fed so as to contribute to formation of a high gain beam. Power is distributed to each of the elements via a feed network, which is the antenna's primary source of inefficiency. It is well known that large feed networks with corresponding large line losses, significantly reduce antenna efficiency.
The above-described arrays are low-profile but suffer in efficiency due to the heavy losses in the feed network. This increases the required array size for a given gain requirement, but the nature of these feed networks is that feed losses become more significant as array size increases. This makes achieving efficient large arrays very difficult. Furthermore, the bandwidth of the above-described arrays is limited by the bandwidth of the elements employed; if a narrowband element such as a simple microstrip patch is used, the array bandwidth is no broader than the bandwidth of each element.
Another approach currently employed is similar to the above-described array, but stacked microstrip patches having dielectric layers therebetween are used instead of simple microstrip patches. The stacked microstrip patches alleviate bandwidth limitations inherent in the previously described array antenna by providing a broad bandwidth element. Stacked patches are well known in the art and comprise two or more patches stacked on top of each other. Each successively higher patch is smaller than those below and centred over the patch immediately below it. Each smaller patch uses the one beneath it as its ground plane, and radiates around the patch above. This technique broadens bandwidth, but does not increase gain, as the patches all have similar radiation characteristics. Bandwidths achieved using this technique can reach 40%.
Arrays of quad-patch elements differ from the previously described arrays in that an array element comprises a quad-patch element in the form of a sub-array fed by a single patch element below each of the patch elements in the sub-array. The quad patch element consists of a first patch which then parasitically couples to four patches disposed above the first patch. A single corner and/or edge of the first patch drives or feeds each patch of the four patches. This reduces feed network complexity and feed network losses, because each group of four radiating patches is fed by a single feed network line.
The use of the quad-patch element provides broad bandwidth, though to a lesser extent than, for example, a stacked patch. A bandwidth of around 15% is achievable. The feed loss problem is significantly reduced due to the larger size and associated higher gain of the quad patch element. The four patches are fed by directly coupling to the first patch--the first patch couples parasitically to the upper four patches. Unfortunately, this configuration is a compromise providing too little bandwidth and insufficient efficiency when placed in large arrays. Also, it is incapable of significant expansion because the feeding technique--one-corner and/or edge-feeds-one-patch--is limiting.
Another issue in antenna design is isolation. It is desirable to provide an antenna capable of radiating two signals that are isolated one from the other. Unfortunately, using conventional patch antenna designs as described above, isolation is insufficient for many applications.